The first elongated bone (metacarpal) at the base of each finger is connected to a proximal phalangeal bone through the metacarpal-phalangeal (MP) joint. This particular joint can be flexed or extended independently of the proximal or distal interphalangeal joint. This variable reciprocal motion, along with the opposability of the thumb, allows for the grasping of objects and the performance of daily functions which are of critical importance to humans. Damage to the MP joint through physical injury or disease can therefore be a severe physiological burden to inflicted humans.
Rheumatoid arthritis (RA), degenerative arthritis, and post-traumatic arthrosis of the MP joint cause interminable pain and poor function of the finger. Patients who have mild symptoms often respond to rest, immobilization, non-steroidal anti-inflammatory drugs, or intra-articular injections of steroids. However, patients who have more severe forms of arthritis may require total joint replacement of the MP joint.
The most common deformity in patients suffering from RA is induced by synovitis of the MP joint which often causes a narrowing of the articular cartilage of the MP joint and attenuation of the collateral ligament structure. The result is often palmar subluxation-dislocation of the proximal phalangeal bone which is caused by a laxity of the flexor complex on the palmar aspect of the MP joint. After loosening of this flexor complex, the action of the flexor tendons provides a dynamic force that palmarly subluxes-dislocates the proximal phalangeal bone. In addition, there is often a secondary loss of cartilage height by erosion and frequently a secondary change in bony architecture, producing a flattening of the metacarpal head and erosion of the dorsal lip of the proximal phalanx. The usual solution is installation of a MP prosthetic joint, see Linscheid et al., "Total Joint Arthroplasty", Mayo Clin. Proc., 54:516-526 (1979); however, in such a case, there is a need for a replacement joint which resists subluxation-dislocation of the proximal phalangeal bone in the palmar direction.
A second important design consideration for MP prosthetic joints is minimizing the wear between the mating articulating surfaces. Mating surfaces may conform to such an extent that biological fluids which would normally provide joint lubrication are expressed from the MP prosthetic joint. The resulting "dry joint" may experience increased friction between the congruent articulating surfaces, as well as create an uncomfortable grinding and/or "squeaky" sensation to the recipient. Increased friction between the congruent articulating surfaces may result in increased wear of the MP prosthetic joint, thereby decreasing the service life of the prothesis. A MP prosthetic joint should preferably avoid exclusion of biological fluid from the congruent mating articulating surfaces.
Another important design consideration for MP prosthetic joints is providing a generally free path for the collateral ligaments which run along each lateral side of the MP joint and for the palmar ligaments or plate. The collateral ligaments of the MP joint comprise both fan-like collateral ligaments and cord-like collateral ligaments. The fan-like collateral ligaments insert at both sides of the distal portion of the first metacarpal bone and serve to support the palmar plate (sometimes called the volar plate) which attaches to the volar aspect of the proximal phalanx and forms a part of the MP overall joint. The cord-like collateral ligaments also insert in shallow depressions at both sides of the dorsal aspect of the distal portion of the first metacarpal bone and cross the MP joint to insert at the lateral volar sides of the proximally phalangeal bone. The cord-like ligaments are slack in extension of the MP joint, allowing for radial-ulnar motion, and are taut during flexion motion, prohibiting radial-ulnar motion. Proper MP prosthetic joint design should have concern for such ligaments.
Accordingly it is the object of the present invention to construct an improved MP joint prosthesis which allows essentially original and natural function to be restored to a damaged finger. To restore natural motion to a damaged finger, the MP prosthetic joint design should provide a free path for the collateral ligaments to run from the distally facing dorsal articular portion of the first metacarpal bone to the proximally facing palmar articular portion of the proximal phalangeal bone.